Automatic guided vehicles have been used for many years to transport goods in warehouses, machine shops, assembly facilities and the like. These vehicles normally have guidance, navigation, and vehicle control systems which enable the vehicle to maneuver through the facility with a relatively high degree of accuracy. These guidance systems often have dead reckoning capabilities which assists the vehicle to operate autonomously and without the aid of any external means such as stripes or wires on the floor.
Dead reckoning systems normally include a steer angle transducer such as an encoder to measure the steering angle of the vehicles steerable wheel and a ground engaging wheel transducer such as a resolver for measuring travel of the vehicle along the underlying surface. These two transducers provide information to an onboard guidance computer which processes the information in accordance with preprogrammed instructions related to a vehicle travel plan such as; heading, speed, and the like, and controls the operation of the vehicle steering and drive control systems. Such dead reckoning systems typically provide acceptable operation for short distances. However, in more complex and long run applications there is a need to provide information to the onboard computer to facilitate corrections and return the vehicle from the actual path of travel to the planned path of travel. This deviation from the planned path of travel is caused by anomalies in the operation of the system. For example, slipping or wear of the ground engaging or steerable wheels, unevenness of the underlying surface of operation of the vehicle and other such factors.
One approach to accurately determining vehicle location is disclosed in U.S. Pat. No. 4,647,784 to P. E. Stephens, dated Mar. 3, 1987. This patent teaches the use of a plurality of reflective bar coded targets placed at predetermined spaced apart locations within an area of operation of the vehicle. The targets are read by a laser scanner and through triangulation the measured location and heading of the vehicle within a particular area of operation is determined. This information is used to make minor corrections to the path of vehicle travel. Because the coded targets are distinguishable from each other the processing of data is accelerated.
U.S. Pat. No. 4,727,492 to P. J. Reeves et al. discloses an automatic guided vehicle having a combination of the laser scanner for reading a plurality of spaced identifiable retroreflective bar coded targets and a dead reckoning system for guiding the vehicle as it travels along a preselected path stored in the memory of a guidance computer. The measured position of the scanned targets is used to correct the estimated position and heading of the vehicle as determined by dead reckoning. The Kalman filter process is utilized to provide the estimated vehicle position. Because the targets are distinct the data processor is able to disregard reflections from other objects or other targets and concentrate processing on the relevant information. This requires that each target be of a substantial size, a size capable of displaying a predetermined number of strips of spaced reflective material of adequate width. Also, placement of the coded targets must be carefully controlled such that two targets with the same code cannot be within range of the scanner at the same time.
As data processing speed and memory capacity have increased over the years attempts have been made to provide a laser scanning navigation system with anonymous reflective targets (non-coded targets). One such system is disclosed in U.S. Pat. No. 4,811,228 to Kaievi Hyyppa dated Mar. 7, 1989. In Hyyppa the vehicle utilizes sensors on the wheels and steering mechanism to measure steering angle and distance traveled which is processed by the onboard computer to calculate the estimated change in position and heading of the vehicle. Hyyppa also discloses a plurality of single strip targets arranged at preselected locations within the area of operation of the automatic guided vehicle. The position of each target is stored in a land base computer and downloaded to an onboard computer of the vehicle as points. Initialization of the vehicle is accomplished, for example, by locating the vehicle at a predetermined position and heading relative to the points (targets). This position is a basis for subsequent calculations and target-point association. The laser scanner scans the area and senses the position of the sighted targets based on an angle measurement of the reflected light relative to the vehicle (heading). The onboard computer, using conventional trigonometry, calculates the position and direction of the truck based on the sensed position. Since the targets are anonymous and there may be more than 3 observed targets and other non-target reflections it is necessary to determine if the reflection came from one of the targets and which target it came from. This verification is called target association. This association utilizes angle measurements of the scanner based on the previous position and heading of the truck. Because there are many targets and the association between targets and points takes a substantial amount of processing time the accuracy of operation of the vehicle may be adversely affected. Further the dynamics of movement of the vehicle further complicates the point-target association. Even in view of the recent advances in computer technology and the speed of operation of such computers complete data processing is not always possible.
Should there be several targets somewhat aligned along the sight line of the laser scanner, clumped together in different facing directions or overlapping the aforementioned association by sensing, the angle of reflection of the laser signal does not provide enough distinction between points (targets) to enable an accurate target-point association to be made. As a result, a substantial amount of processing time may be spent in the association without any confidence that the results are accurate. This inaccuracy results in additional vehicle position and heading error which further compounds the above identified error.
Further inaccuracy is built into the system based on error in the scanner, error in the scanner angle sensors, steering angle and travel distance error and the like. This error affects the position and heading calculations of the vehicle and causes additional errors in the navigation of the vehicle.
The present invention is directed to overcoming one or more of the problems as set forth above.